Wolf Sarah E, Fulco Sage, Zhang Aixi, Zhao Haoqiang, Walsh Patrick J, Turner Kevin T, Fakhraai Zahra
J Phys Chem Lett. 2022 Apr 21;13(15):3360-3368. doi: 10.1021/acs.jpclett.2c00232. Epub 2022 Apr 11.
The density, degree of molecular orientation, and molecular layering of vapor-deposited stable glasses (SGs) vary with substrate temperature () below the glass-transition temperature (). Density and orientation have been suggested to be factors influencing the mechanical properties of SGs. We perform nanoindentation on two molecules which differ by only a single substituent, allowing one molecule to adopt an in-plane orientation at low . The reduced elastic modulus and hardness of both molecules show similar dependences, with enhancements of 15-20% in reduced modulus and 30-45% in hardness at ≈ 0.8, where the density of vapor-deposited films is enhanced by ∼1.4% compared to that of the liquid-quenched glass. At < 0.8, one of the molecules produces highly unstable glasses with in-plane orientation. However, both systems show enhanced mechanics. Both the modulus and hardness correlate with the degree of layering, which is similar in both systems despite their variable stability. We suggest that nanoindentation performed normal to the film's surface is influenced by the tighter packing of the molecules in this direction.
气相沉积稳定玻璃（SGs）的密度、分子取向程度和分子分层随低于玻璃化转变温度（）的基底温度（）而变化。密度和取向被认为是影响SGs力学性能的因素。我们对仅相差一个取代基的两种分子进行纳米压痕测试，使得一种分子在低温下呈现面内取向。两种分子的约化弹性模量和硬度表现出相似的 依赖性，在 ≈ 0.8时，约化模量提高15 - 20%，硬度提高30 - 45%，此时气相沉积薄膜的密度相比液体淬火玻璃提高了约1.4%。在 < 0.8时，其中一种分子会产生具有面内取向的高度不稳定玻璃。然而，两个体系的力学性能均有所增强。模量和硬度都与分层程度相关，尽管它们的稳定性不同，但两个体系中的分层程度相似。我们认为，垂直于薄膜表面进行的纳米压痕测试受到该方向上分子更紧密堆积的影响。
